I would have thought this is the kind of thing only a garage could help with.
However, the CO sensor, presume you mean the oxygen sensor in the exhaust? Dont think this is the problem, it will cause the engine to run rough but not stop it firing up. By the way, dont ever push start your car, you will damage the catalytic converter!!!!
It sounds like the ignition/fuel/electrical system.
Are you getting sparks to the engine everytime or just occassionally?
Perhaps a fuel problem, check the pump and filters.
Is there a bad earth for the car?
The list could go on and on!!!
Also, the price you have been given for the Oxygen sensor is very expensive, they are about 80-100euros here. Don't know about the RPM sensor.
Try another garage!